Using the HDMI handshake or conversation period might be a way into the 360. It is so tightly controlled in specification that a lot of designers had to make sacrifices to comply even with 1.1 specification.
I can't find any information on common software approaches used in HDMI or the complete detailed specification, hdmi.org has it but for manufacturers only. As Microsoft are retrofitting HDMI does it already exist on the motherboard?. Bcakwards compatibility is compulsary so maybe there is an existing method ?, I don't think anyone has tried deliberate buffer overflows through HDCP or just HDMI.
TIVO firmware buffer overflow when they changed HDMI firmwares 6.4 I thnk
Toshiba HD-A1 crashes when talking to display and TV does not report being in the ON or Standby state.
Some Cable boxes get confused when two devices talk back
HDCP which is optional, might have a handshake bug for multiple requests or a back off period.